The present invention is related generally to radio frequency (RF) signal multiplexing apparatus, and more particularly to improved RF signal combining/sorting apparatus that is particularly well adapted for use in antenna duplexers for mobile, portable and fixed station radios.
Conventional RF signal duplexers typically consist of a conductive enclosure having two filters each comprised of a plurality of cells including a resonator of the helical type as shown and described in U.S. Pat. No. 3,728,731 or the coaxial type. The helical and coaxial resonators are essentially foreshortened short-circuited quarter-wavelength transmission lines. The resonators may be inductively coupled one to another by apertures in their common walls. Each resonator can be tuned by means of a tuning screw which inserts into a hole extending through the middle of the resonator. Once tuned, the overall response of the duplexer is determined by the size of the interstage coupling apertures. Since the tuning of the resonantors can be disturbed by a slight adjustment of the tuning screw, a lock nut is required to keep the tuning screw properly positioned at all times. The use of tuning screws not only renders the duplexer susceptible to becoming de-tuned, but also creates additional problems including mechanical locking of the tuning screw and arcing between the tuning screw and the resonator structure. Furthermore, these duplexers tend to be rather bulky, and therefore are relatively unattractive for applications where size is an important factor.